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In an electrical circuit, energy is transferred each second. This is electrical power. The power output of a transformer can be calculated using the equation:

power = potential difference × current

This is when:

• power is measured in watts (W)
• potential difference is measured in volts (V)
• current is measured in amperes, also referred to as amps (A)

Assuming that a transformer is 100 per cent efficient, the following equation can be used to calculate the power output from the transformer:

potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil

Example

A step-down transformer converts 11,500 V into 230 V. The power output is used to run a 2,000 W kettle. Calculate the current in the primary coil (the input current).

(The power in each coil will be the same, since transformers are 100 per cent efficient.)

Rearrange the formula to find current:

The National Grid

When electricity is transmitted around the country on the National Grid, the transmission wires get hot and energy is dissipated to the surroundings.

Step-up transformers at a power station increase the voltage which reduces the current. The smaller the current, the less energy is dissipated so stepping-up the voltage makes the transmission of electricity more efficient.